1 set of resistors is probably adequate, however, the value of the Rs will be determinedby how many devices are on the buss, and how long the wires are. This is because all thosedevices and longer wires increase the total capacitance on the buss, so the risetimes ofthe signals will increase [simple first-order RC charging]. If this gets too long, buss operation will become irratic.

Whereas you usually see 4.7K - 10K pullup values mentioned, 2.7K is probably a betterchoice for the situation mentioned above.